Vaccines that induce antibody-mediated immunity have drastically improved human health. However, successful vaccines exist for only a fraction of pathogens and even the most successful vaccines exhibit drastic variability. For example, some vaccines induce levels of protective antibody that varies 10,000-fold from person-to-person. Another example of variability is that some vaccines induce long-lived immunity while others require life-long re- vaccination. The key to understanding these differences in protective immunity is to understand the development and persistence of the long-lived lymphocytes that confer protection. Antibody-mediated immunity relies on the proper development and persistence of long-lived "memory" B cells and antibody-secreting plasma cells. Unlike plasma cells, memory B cells respond to secondary pathogen encounter and differentiate quickly into antibody secreting cells. Despite this importance in protective immunity, the mechanism of memory B cell differentiation is not well understood. The objective of this proposal is to identify and characterize developing memory B cells. This is important because access to developing memory B cells will allow us to identify molecules important for this transition that are not readily apparent from the study of fully developed memory cells. Memory B cells are the long-lived, quiescent progeny of cell death-susceptible, proliferating precursors. It is unknown how or when developing memory B cells exit cell cycle and upregulate an anti-apoptotic program. The central hypothesis of this application is that "memory precursor" B cells can be identified as non-quiescent cells that have adopted the survival program and cell surface phenotype of bona fide memory cells. Therefore, my specific aims are: 1. Determine when quiescent memory B cells arise from proliferating precursors. And 2. Determine when developing memory B cells upregulate a survival program that allows these cells to avoid cell death. The proposed studies are innovative because cutting edge enrichment techniques will be utilized to analyze the development of memory B cells without the aide of artificially increasing the starting number of cells. At the completion of these studies I will have a complete understanding of the kinetics of memory cell development and will have begun to probe the role of candidate molecules in this process. PUBLIC HEALTH RELEVANCE: Vaccination aims to induce the development of long-lived cell populations that confer protection against the select agent (pathogen/protein/etc.) of interest. In this proposal I will probe the functional development of one of these long-lived populations to determine the mechanism by which these cells develop. The long-term goal of this work is to understand the development of these cells such that vaccines can be designed to maximize protective responses.